The invention relates to a data bus for a plurality of nodes, which are connected to one another via a star coupler. Such a data bus is known from the previously unpublished German patent application 19720401. The nodes are connected to the data bus via send/receive modules. Particularly if nodes sending optical data messages are connected to the data bus, there may be a malfunction of a bus node (e.g. continuous sending of messages) or an error in a send/receive module (e.g. output of an optical/electric transducer used in the send/receive module remaining on the low level that normally occurs only during signal traffic, even when there is no signal traffic) which blocks bus communication. In this case the error source cannot be determined automatically. Nor is it possible to intervene automatically. Bus communication remains down until repair.
The object of the invention is to create a data bus which makes it possible to monitor the data traffic on the data bus and, when a malfunction occurs, to identify the source.
The present invention provide a series of individual measures that in combination have the desired effect. On the one hand, any optical signals are converted into electrical form and are supplied to the star coupler as input signals in electrical form. The star coupler itself contains a logic decision gate to which the input signals are routed and the output of which is connected in parallel to the inputs of the nodes via an electrical line. A storage element is connected in parallel to each input of the decision gate and permanently changes its state when an input signal occurs at this input.
The addressability of the storage element makes it possible in a simple manner to detect which node(s) was/were involved in the bus traffic or at which input an error exists. For this purpose, the state of the storage element can be read out, for example, by a microcontroller assigned to the star coupler via a serial interface (e.g. PI). If a fault occurs during message transmission, the source of the fault can thus be determined.
Finally, the storage element can be reset after an error free transmission or after being read out by the microcontroller. Any subsequently occurring error can be detected and distinguished from the previous error.